Arugula and the bitterness trap: Arugula naturally contains mustard oils and bitter compounds that give it its characteristic peppery taste. If the Vitamix runs too long on a high setting, the bitter note of the olive oil adds to this — the pesto can then turn too sharp. The remedy: first chop the arugula, pine nuts, and Parmesan coarsely on setting 5 to 6 (about 10 seconds), stop the blender, then restart on setting 1 and let the olive oil run in slowly through the lid opening. As soon as the pesto looks homogeneous, stop right away. You can also stir in the last third of the oil by hand — the safest method for this version.
Arugula quality: Younger, small-leaf arugula (baby arugula) has a milder flavor than mature arugula with large leaves. For a robust pesto, use large-leaf arugula; for a milder one, choose baby arugula or mix both. In either case, wash the leaves well and pat them dry so the pesto does not turn watery.
Why the Vitamix: Arugula leaves contain mustard oils that often stay only partly broken down in a mortar, leaving the pesto fibrous. In the Vitamix, setting 5–6 chops the leaves completely in about 10 seconds before the olive oil runs in on setting 1 as a stable emulsion. This produces a smoother, more matte texture than any immersion blender manages. Separating the stages matters here in particular, because it keeps the arugula's natural bitterness and the olive oil's polyphenols from compounding into a bitter result.